Field of the Invention
The present invention relates to stringed musical instruments having a truss rod for adjusting the curvature of the neck and compensating for bowing of a neck due to string tension.
Background
Modern guitars consist principally of a body, a neck and a peghead. The neck extends from the body to the peghead. Guitar strings extend from the peghead to the body and are under tension when in tune. The guitar strings extend above the top surface of the neck and body and, when tensioned to pitch, exert a force that has a tendency to bow the neck in a concave manner in relation to the top surface of the neck.
To offset this tendency of the neck to bow, a metal rod that is bent into a concave shape is frequently embedded within the neck. One end of the metal rod is restrained within the neck, typically by a 90 degree hook, and the other end typically is threaded with a nut. The length of the truss rod is typically covered with plastic that prevents it from sticking to the wood when being glued in place. When the nut is tightened against the neck wood, the tension in the truss rod acts to counter the force exerted by the strings to prevent bowing of the guitar neck.
The adjustment of the curvature of the neck is made by tightening or loosening the nut at the threaded end of the truss rod against the wood of the neck. Many truss rods function only in one direction to offset the bow in the neck that occurs from the tension of the strings. However, it is desirable to have a slight concave bow in the neck in order to prevent strings from buzzing on the frets when the action (distance between the fret and string) is low. If the string tension is not sufficient to provide this slight concave bow, then some guitars have two-way truss rods that allow this bow to be produced. There are many designs for two-way truss rods, but most employ two metal rods (instead of one) interacting against each other. These designs usually require a deeper truss rod channel but are adjusted in a similar manner to a one-way truss rod.
The truss rod nut can be located at either the peghead end or the body end of the guitar neck. Since the truss rod is embedded in the neck, a slot or notch must be cut into the peghead or the body to access the truss rod nut. Because the strings also run generally in the same direction as the truss rod, maneuvering a wrench through the slot, into the truss rod nut and rotating it can be challenging. When the adjustment is made at the peghead, removing the neck material to access the truss rod nut can weaken the peghead strength making it more prone to breaking if dropped. Truss rod adjustments at the body end are difficult to make because the wrench must be manipulated through the strings or even require the removal of the strings or neck to make the adjustment. This makes correct adjustment difficult.